In the first few pictures you posted his top line actually looks pretty good. Are you concerned about the area on the sides of his withers, behind his scapula? If that’s what you’re talking about, that is only fixed through an appropriately fitting saddle. I would make sure the saddle you have fits him well, because the dips behind his scapulas looks like it doesn’t.
im not worried about that, his saddle is new and he was fitted properly. im more worried about his hindquarters and was wondering if the balance issues stemmed from a poor topline
The horse has a less than ideal.topline from withers to tail. It’s all part of whatever’s going on.
I don’t think his top line is so weak that he obviously couldn’t canter in balance. It is however possible that there is something neurological going on that contributes to both muscle wasting and to lack of balance.
How do you know his saddle was fitted correctly?
I’m not trying to berate the issue (or you!) but that dip behind the withers means the saddle doesn’t fit. Now, if that dip has been there forever and you just got the new saddle two weeks ago, maybe the new saddle does fit. However, it’s incredibly hard (near impossible) to fit a saddle when those dips exist, and the horse really should not be ridden until that muscle has time to recover.
If you are working with a trainer that has experience with long lines, I’d longe (not with side reins) and long line this horse for a month minimum (starting with light work, only walk trot, working up to cantering and cavalettis) to allow his back to recover.
we had a professional do it and my trainer helped as well
it’s fine, i asked for help and you’re giving it to me! my trainer has never mentioned this dip and he’s never been sensitive along his spine. i do know he’s had that since we bought him (not sure if that helps at all)
Here’s a post that gives some more specifics: https://www.instagram.com/p/CyNhslGMBFL/
Really the whole account is quite educational about saddle fit. 
I have seen enough badly fitted saddles by socalled professionals and also by trainers.
How old is he?
I am referring to the photo from the rear and the second one from his side (I love his color). The top of scapulae jut out way more than usual, which I have not seen except when a horse is OLD or nearly starving to death. Your horse is definitely getting enough food and he looks well cared for.
Be extremely careful with your saddle. Horses refuse to move as freely when the top of their shoulder runs into the flap of the saddle and the points of the saddle tree. I see those pictures and I would be thinking more of using a decent bareback pad rather than a saddle.
The next time you get your vet or body worker out ask him/her to check the trapezius muscle on both sides. The trapezius muscle covers the top of the scapula and the scapular cartilages on the top end of the shoulder bone. I should not be able to see anatomical details of the top of the shoulder bone on a horse that is in good weight. His high withers should make saddle fitting very challenging. (All of this comes from looking at horses and horse pictures closely for over 60 years and studying horse anatomy for most of this year, lots and lots of anatomy books! I am not a vet.)
I am so glad that you care about your horse. There is much I like about him.
Meh I rode a VERY nice jumper in “his” very expensive custom saddle - fit was atrocious and he had the biggest wither dips/hollows. I’m no fitter or vet but I guarantee a better fitting setup and some lunging/rehab work would’ve done him a world of good. His connections swore that’s “just how he is”.
For all of my one gelding’s theatrics about EVERYTHING - this horse barely did more than pin his ears at saddling.
Sadly, while many trainers are great at training horses and even teaching riders how to ride (and sometimes teaching riders how to train their horse), too many have no idea about conformation, or muscle development, let alone saddle fit.
Those hollows beside the withers are not healthy muscle. That’s a separate issue from what caused them, whether the current saddle truly fits, etc. It’s disheartening that professionals in the horse world still can’t see this is a problem
Your saddle may indeed fit but it could be placed too far forward. This may not be the situation in your case but I cannot tell you how many trainers I have seen who insist on the saddle being too far forward. It can be a matter of inches.
The number of geldings I’ve ridden that just suck it up about poor saddle fit (or are yelled at for showing their discomfort and now have shut down) is…a lot. It’s incredibly sad and makes me always second guess tack fit even if the horse isn’t showing pain signals.
My mares would never 
There are two main muscles that cover the outside of the scapular cartilage at the top of the scapula. There is the aforementioned Trapezius thoracica which descends from the nuchal ligament over the top of the scapula, and there is the Latissimus dorsi muscle which descends in a broad sheet from the thoracumular fascia on the back of the horse under the saddle, narrows as it angles down and to the front to end on inside of the humerus. The Latissimus dorsi goes OVER the scapular cartilage, so there are two muscles that go over the top of the shoulder bone.
From what I am reading it really pays to work on these muscles first since they tie in further back on the horse’s back.
If you can ask your vet or your equine body worker to suggest exercises you can do with your horse to strengthen these muscles. Wastage up here can be caused by improper saddle fit, which has happened in the past or he would not have white hairs at the top of his withers. A saddle that “fits” now will NOT fit when you get these muscles stronger, which is why I suggested a bareback pad (a good one, preferably with someway to keep it off the horse’s spine) set back from the withers and the top of the shoulders if you want or need to ride your horse.
Good for you for correctly identifying that your horse has muscle problems on his topline. Good for you for wanting to fix the problem instead of just shrugging it off. Getting these two muscles stronger and thick enough so that they can fulfill their anatomical role and support the front of the saddle so that it will clear the top of the withers will be a good start at improving your horse’s topline. As a plus it should make your horse a much better riding animal.
First, you clearly care about your horse and you’ve identified an issue and are asking questions to get help. So you’re already heading in the right direction in my opinion. Second, I’m not an expert in this by any means, but just by the way your horse stands I wonder if he’s not the example of a weak thoracic sling my body worker tries to explain to me, and that can explain everything you described. I’d also double check saddle fit as I’ve seen professionals claim a saddle fits… when it doesn’t, which I experienced first hand early on in my horse owning journey. In terms of increasing topline-- make sure you’re feeding quality hay with sufficient protein. If that’s in check, you can add some gamma oryzanol or rice bran oil to help aid muscle development. If he still doesn’t develop muscle and has muscle wasting you can have a neuro exam or check for cushings, which can cause the same thing where the horses struggle to put on muscle. (https://ker.com/equinews/pergolide-and-muscle-atrophy-in-ppid-horses/)
That’s exactly what I was thinking. You’ve got to fill that in (what’s worked for me was amino acids - Tri Amino by Uckele worked wonders) if you can’t confirm you are providing the three basics: Lysine, Threonine and Methionine.
I’d not be riding and instead do a through diet analysis - exactly how much protein - in grams - are they getting and work on the lunge and build a back muscle. Long line, drive, anything just don’t sit on that back till you’ve got muscle. Maybe it’ll be just 30-60 days.
And it will pay off in the long run. Think long term. 
he’s only six
